1. Field of the Invention
The present invention relates to fluid distribution manifolds, and more particularly to high temperature fuel manifolds for gas turbine engines.
2. Description of Related Art
The fuel manifold system of a gas turbine engine distributes fuel from a fuel control system to a plurality of fuel injectors mounted on the engine case. The fuel injectors are configured to issue atomized fuel into the combustor of the engine. It is well known that combustor inlet air temperatures can be extremely high (e.g., 1300° F., 704.4° C.), and the combustion of fuel drives temperatures even higher. High combustor temperatures are necessary in order to fully ignite the fuel and to derive the maximum amount of energy available from the burning fuel in a turbine, and often a nozzle, located downstream of the combustor. Under basic thermodynamic principals, increasing the temperature and/or pressure of combustion gases increases the amount of useful energy that can be produced. As gas turbine engine technologies have advanced, higher and higher operating temperatures have become possible, making for increasingly powerful and efficient engines.
The high operating temperatures in modern gas turbine engines put a tremendous thermal strain on engine components associated with the combustor casing. These engine components must structurally accommodate thermal expansion and contraction of the combustor casing during engine operating cycles. There is a particularly high gradient of thermal expansion at the fuel manifold, where the internal fuel temperature is relatively low compared to the external gas temperatures. A combustor easing is typically around room temperature prior to engine start up, and then heats up to a high operating temperature during high power engine operation, such as during takeoff. While the combustor casing expands and contracts considerably with these thermal cycles of the engine, the fuel manifold undergoes comparatively little thermal expansion due to the relatively cool fuel flowing through it. This cycling difference in thermal expansion between the engine case and fuel manifold must be accommodated to avoid stress related failures.
Traditionally, thermal expansion of the combustor case has been accommodated by using curved metal tubes to flexibly connect the fuel lines of the manifold assembly to the fuel injectors. An example of a fuel manifold assembly that includes curved metal tubes is shown in U.S. Pat. No. 5,197,288 to Newland et al. Prior art designs of this type have certain disadvantages such as susceptibility to vibration and fatigue.
Another solution has been to use flexible hoses to connect between injector fixtures of the manifold assembly to accommodate thermal expansion. The United States Military Defense Standards MIL-DTL-25579 establishes an upper limit of 450° F. (232.2° C.) for the air around a flexible fuel manifold in a gas turbine engine. However, today's high performance gas turbine engines have air temperatures outside the engine case that far exceed that standard. Typical fuel systems are expected to be able to routinely operate in temperatures in excess of 800° F. (426.67° C.).
Additionally, the Federal Aviation Authority (FAA) requires that commercial engine fuel systems undergo a flame endurance test to ensure that the fuel systems can safely operate even under prolonged exposure to flames. Exposure to flames can compromise fuel manifold hoses. It is known to provide a fire sleeve around the hose portions of such manifolds to improve flame resistance. However as gas turbine technology advances, the operating temperatures continue to rise to levels that can compromise even hoses with conventional fire sleeves.
One solution to this problem has been to utilize a telescoping outer wall outside the fire sleeve, as disclosed in U.S. Pat. No. 4,467,610 to Pearson et al. While this may improve the ability of a manifold to endure high temperatures and flames, such mechanisms add to the cost and mechanical complexity of flexible hose type fuel manifolds.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for fuel manifolds that allow for improved high temperature operation and flame resistance. There also remains a need in the art for such manifolds that are easy to make and use. The present invention provides a solution for these problems.